<!DOCTYPE html>
<html class="reftest-wait reftest-no-flush">
<title>
  Opacity animation creates a stacking context after changing effects
</title>
<style>
span {
  height: 100px;
  width: 100px;
  position: fixed;
  background: green;
  top: 50px;
}
div {
  width: 100px;
  height: 100px;
  background: blue;
}
</style>
<span></span>
<div id="test"></div>
<script>
  var elem = document.getElementById("test");
  var anim = elem.animate(null, { duration: 100000 });
  var newEffect = new KeyframeEffect(elem, { opacity: [1, 1] }, 100000);
  anim.ready.then(function() {
    anim.effect = newEffect;
    requestAnimationFrame(function() {
      document.documentElement.classList.remove("reftest-wait");
    });
  });
</script>
</html>
